Some historical considerations concerning thermodynamic cycles were already set forth in the description of patent applications MI2013A000040 (pages 1-9) and MI2012A001944 (pages 1-8) in the name of the same Applicant, and it is therefore deemed useful to mention the more significant innovative parts forming the subject matter of the present inventive idea, regarding a new system for the transmission of motion between the pistons and the drive shaft, the use of the drive unit in some further extensions of the Rankine-Hirn heat cycle, the use thereof with a new heat cycle derived from the Stirling cycle and the use thereof in a new compressed air motor.
Theory of the Stirling Heat Cycle
In 1816 Robert Stirling introduced a hot air engine with an open circuit, characterized by an intermittent flow made up of four phases: intake of air at atmospheric pressure (at ambient temperature), compression of the air taken in, rapid heating and expansion of the pre-compressed air and expulsion of the exhaust air (into the environment).
In a subsequent evolution of the Stirling cycle, with the aim of increasing thermal efficiency while maintaining the four-phase cycle unchanged, a bi-directional heat exchanger defined as a “regenerator” was introduced into the circuit; subsequently, multiple solutions with different types of closed-circuit reciprocating and rotary engines were developed.
The basic Stirling cycle is very schematically represented in FIG. 9. It consists of two adiabatic transformations and two isothermal transformations.
The area comprised between the four transformations defining the cycle represents the net work “L” obtained through the cycle. This work is obtained as the difference between the positive work 1−2+2−3 and negative work 3−4+4−1.
The total heat “Q” which the heat engine retains is likewise given by the difference between the heat it has absorbed (Q2) and the heat it has discharged (Q1); therefore, the work that the heat engine is capable of producing is given by:L=Q2−Q1 
Based on the above, we can affirm that only part of the heat supplied to the heat engine is transformed into work, whilst the remaining part is necessarily discharged outside the system. For heat engines, it is thus possible to define efficiency as the ratio between the work the machine produces and the heat necessary to input to the machine, i.e.η=L/Q2 
Essentially, in order to improve thermal efficiency it is necessary to increasingly diminish the quantity Q1/Q2, which is subtracted from the value 1 and which reduces efficiency. For this purpose, the values of the numerator and denominator must be as far away from each other as possible, i.e. the hot source must work at the highest temperature possible and the cold source must be at the lowest temperature possible.
The temperature of the hot source is subject only to technological limitations tied to use, cycle and materials, whilst as regards the temperature of the cold source there are limited possibilities of intervention: in fact, it is typically necessary to use the temperature of the outside environment or that of a coolant fluid made to circulate in a specific exchanger.
Overall, various machines functioning with a Stirling heat cycle have been developed and others are still at an experimental stage. However, the Applicant has found that even already industrialized solutions have limitations and can be improved in several respects. This applies, in particular, for Stirling engines used to drive small and medium power autonomous electric generators (below 50 KWh).
In practice, besides the various types of Stirling engines, the following are presently used to drive electric generators:                reciprocating internal combustion engines, which are mechanically complicated, noisy (and also for this reason are made to operate at a low rpm), are particularly polluting and have high maintenance costs;        gas turbines, which, besides being particularly costly, are not competitive in small-scale applications.        engines using the Rankine or Rankine-Hirn cycle, to which the present inventive idea also relates, but which, given the need to use a steam generator, can be strongly competitive only in fixed cogeneration applications (where it is possible to almost entirely recover the residual heat of the heat cycle), require further technological innovations in order to be profitably used also in small-scale mobile applications.        
In general, all of the prior art solutions, in addition to the problems of pollution, low efficiency, mechanical complexity and high maintenance costs, are also characterized by a cost-benefit ratio that is not particularly satisfactory.
Theory of the Compressed Air Motor Cycle
The engine exploits the energy contained in tanks of compressed air which, according to the ideal gas law, is maximum for an isothermal transformation and is equal to:
  Energy  =                    P        1            ⁢              V        1              ⋆          ln      ⁡              (                              P            1                                P            2                          )            where P1 is the initial pressure of the tanks and V1 is the volume of transformation, equal to the cubic volume of the engine, whilst P2 is the atmospheric pressure.
With pressures that are not too high and temperatures that are not too low, compressed air behaves like an ideal gas with excellent approximation and the energy it contains is thus given by the above formula.
Let us indicate the energy P1V1*Ln(P1/P2) as Emax, the energy of a perfectly isothermal transformation.
Let us indicate as E the mechanical energy that the compressed air motor transforms starting from tanks of volume V2 and pressure P2.
Thus the engine's efficiency is given by:Eff=E/Emax
As in general the energy of the transformation is equal to the area (integral) below the transformation curve in the Clapeyron diagram.
In the case of an adiabatic transformation, the energy, or work, is in fact for this transformation:Eadiab=(P1V1−P2V2)/(g−1)g=Cp/Cv PV9=costwhere Cp for air is about 1.00 and Cv for air is about 0.72. The efficiency is equal to:Eff=Eadiab/Emax